Silicon (Si) is conventionally used as constituent material for power semiconductor devices controlling high voltage and large current. The power semiconductor devices fall into several types, such as bipolar transistors, insulated gate bipolar transistors (IGBTs), and insulated gate field-effect transistors (MOSFET), and are selectively used according to the intended purpose.
For example, bipolar transistors and IGBTs cannot be switched at high speed although higher current density enables larger current as compared to MOSFETs. For example, the use of bipolar transistors is limited up to a switching frequency of several kHz and the use of IGBTs is limited up to a switching frequency of tens of kHz. On the other hand, power MOSFETs can perform high-speed switching operation up to several MHz, although lower current density makes it difficult to handle larger current as compared to bipolar transistors and IGBTs.
However, since a power semiconductor device supporting both large current and high-speed performance is strongly demanded in the market, efforts are made to improve IGBTs and power MOSFETs, which have been developed substantially to the theoretical limits determined by the materials. Semiconductor materials replacing silicon are studied in terms of power semiconductor devices and silicon carbide (SiC) is attracting attention as a semiconductor material usable for producing (fabricating) a next generation power semiconductor device excellent in low ON-voltage, high-speed characteristics, and high-temperature characteristics.
Chemically, silicon carbide is very stable semiconductor material with a wide band gap of 3 eV and can be used extremely stably as a semiconductor even at high temperatures. Silicon carbide has critical electric field that is 10-fold or greater than silicon and therefore, is expected as a semiconductor material capable of making ON-resistance sufficiently small. Such features of silicon carbide are the same as those of other wide band gap semiconductors, for example, gallium nitride (GaN). Therefore, by using wide band gap semiconductors, a higher breakdown voltage of a semiconductor device can be achieved.
However, in a high-voltage semiconductor device, high voltage is applied not only to an active region in which an element structure is formed, but also to a breakdown voltage structure portion disposed in a peripheral portion of the active region to retain a breakdown voltage, and an electric field concentrates on the breakdown voltage structure portion. The breakdown voltage of a high-voltage semiconductor device is determined by the impurity concentration, thickness, and field intensity of the semiconductor, and the withstanding capability is determined by semiconductor-specific features, and in this way, is equal across the active region and the breakdown voltage structure portion. Therefore, when the electric field concentrates on the breakdown voltage structure portion, an electric load exceeding the withstanding capability may be applied to the breakdown voltage structure portion and may cause destruction.
A semiconductor device having a breakdown voltage structure portion provided with a termination structure such as a junction termination extension (JTE) structure and a field limiting ring (FLR) structure is known as a device having a breakdown voltage of the entire high-voltage semiconductor device improved by alleviating or dispersing the electric field of the breakdown voltage structure portion. In a known semiconductor device, a floating metal electrode in contact with the FLR is disposed as a field plate (FP) to release electric charge generated in the breakdown voltage structure portion for improvement in reliability (see, e.g., Patent Document 1).
FIG. 2 is a cross-sectional view of a Schottky barrier diode (SBD) of a conventional technique. A Schottky electrode 17 and an electrode pad 18 are disposed on a silicon carbide substrate 12 and an active portion is formed while a breakdown voltage structure portion (edge portion) is formed from a ring-shaped p-type region 15 to surround the active portion, thereby retaining a breakdown voltage. The p-type region 15 is made up of a p+ type region 15a disposed at the termination end of the Schottky electrode 17, a p-type region 15b adjacent to the outer periphery of the p+ type region 15a, and a p− type region 15c adjacent to the outer periphery of the p-type region 15b. Reference numeral 16 denotes an interlayer insulation film made of oxide silicon (SiO2), etc.
The electrode pad 18 is made of aluminum-silicon (Al—Si) based alloy, for example, and has a thickness of about 5 μm. The electrode pad 18 has such a predetermined thickness for impact absorption at the time of ultrasonic bonding using wire bonding of Al etc.
Fabrication steps of the electrode pad 18 and the Schottky electrode 17 will be described as follows:    1. deposition of Ti by sputtering;    2. resist patterning through resist application and photolithography;    3. wet etching of Ti with ammonia hydrogen peroxide water to form the Schottky electrode 17;    4. deposition of Al—Si by sputtering;    5. resist patterning through resist application and photolithography; and    6. wet etching of Al—Si with phosphoric/nitric/acetic-acid mixture liquid to cover the Schottky electrode 17. In this case, Ti is not wet-etched by the mixture liquid. Subsequently, to remove remaining Si (called nodules), dry etching (particle etching) is performed by using carbon tetrafluoride and oxygen as source gas to form the electrode pad 18.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2010-50147